1. Field of the Invention
The present invention relates to a power supply device connected to an AC power source for generating a DC voltage, and more particularly to a power supply device that reduces power loss.
2. Description of Related Art
In recent years, environmental problems, such as global warming, have attracted public attention, prompting interests in the reduction of power loss in power supply devices, such as AC adaptors. In response, certain AC adaptors connected to commercial alternating current (AC) power sources having reduced power loss during waiting time have been marketed. For certain conventional products, ranges for such reduction in loss have gone from 3 W to about 0.5 W.
Published Unexamined Patent Application No. 8-205399 discloses a technique for sensing the presence of load from the amplitude of the output voltage from an AC adaptor, and reducing power consumption by stopping the operation when the output side is in no-load state. Published Unexamined Patent Application No. 2000-308257 discloses a power source circuit that is constituted so as to activate equipment to which power is supplied from a power source circuit by starting the supply of power on receiving an output from the internal power source provided in the equipment, and to stop power supply completely when the equipment is in the standby state.
As in the above-described AC adaptors, although power loss in the standby state can be reduced to about 0.5 W in recent years, further reduction of power loss must be attempted when environmental problems, such as global warming, are considered. Particularly, it is very desirable from the point of view of energy saving to significantly reduce power consumption in waiting time when the apparatus is not operated (when the main apparatus is not connected).
However, in PCs (personal computers) or note PCs (notebook type personal computers), for example, since the main body of the apparatus must be maintained in a soft-off state in order to operate the main body, for example, when an external signal comes in, even if the main body is in a power-off state, there is often a problem to turn the output off completely. Also, for example, while notebook PCs and desktop PCs have a suspend function of halting the machine (while maintaining the operating state of the program), it is often also the situation where a reduction in power loss is needed when only the bare minimum power needed for maintaining the data is available.
Also in the technique disclosed in the above-described Published Unexamined Patent Application No. 8-205399, the presence of load is sensed from the amplitude of the output voltage from the AC adaptor, and, as a result, the presence of load cannot be sensed from the amplitude of the voltage when the current is very small during the soft-off state or the like. In order to sense a very small current, a current sensing resistor that has a high resistance is used, and a large voltage drop occurs in a maximum load. Therefore, it is difficult to constitute a practical AC adaptor that maintains the output voltage constant using such a technique.
Furthermore, according to the above-described Published Unexamined Patent Application No. 2000-308257, since the output is completely turned off, the method is not applicable to a soft-off or suspend state introduced in note PCs or the like. Also, there is a problem that the AC adaptor cannot be turned on any more, when the battery is disconnected, or the battery is dead.